Regarding pigment dispersion compositions, such as paints, printing inks, and makeups, the required levels and types of their functions and performances have increased and the technical challenge for stably dispersing a substance to be dispersed has become greater. For example, a pigment can more sufficiently exhibit performances and characteristics inherently possessed by its particles themselves with increasing degree of fineness, but the achievement of its sufficient dispersibility often cannot be made in combination with a vehicle alone and further requires a dispersant. Dispersion compositions famed of fine particles generally exhibit high viscosity, which not only makes it difficult to take out their products from dispersers and transport them but also, at significantly high viscosity, may cause gelation during storage and thus make it difficult to use the products.
The dispersant has a structure including a part adsorbable to a pigment and a part having a high affinity for a dispersion medium and the performance of the dispersant is determined by the balance between these two functional parts. Various types of parts adsorbable to a pigment can be used according to the surface condition of the substance to be dispersed; an acidic part (acidic dispersant) is used for a pigment having a surface biased toward the basic side, while a basic part (basic dispersant) is used for a pigment having a surface biased toward the acidic side.
Meanwhile, in producing a color filter for use in a liquid crystal display or the like, a coating film made of a dispersion composition in which a pigment, a dispersant, and a dispersion medium are mixed is formed on a substrate and exposed to light through a photomask of a desired pattern shape and unexposed portions thereof are removed with an alkaline aqueous solution (alkaline development). In this case, to remove the unexposed portions with an alkaline aqueous solution, a developing polymer with an acid group is added to the dispersion composition. During the alkaline development, the unexposed portions are removed by neutralizing the acid groups of the developing polymer with the alkaline aqueous solution to turn the developing polymer water-soluble. Therefore, basic dispersants, which are less soluble in alkaline aqueous solutions, cause deterioration in developability.
Patent Literature 1 proposes to use as a pigment dispersant an AB block copolymer composed of an A polymer block and a B polymer block and containing 90% by mass or more structural unit derived from a methacrylate monomer, wherein at least the B polymer block of the polymer blocks forming the copolymer contains at least one of two types of acid groups present in a carboxyl group-containing methacrylate monomer having a carboxyl group and a phosphoric acid group-containing methacrylate monomer having a phosphoric acid group.